The present invention relates thus to the device for treatment of wastewater which contains biologically degradeable organic material by means of anaerobic fermentation. Treatment of organic material is performed by both facultative and obligate anaerobic bacteria during anaerobic fermentation. Carbohydrates, proteins and lipids are at first hydrolized and thereafter these hydrolysis products are further digested mostly into acetic acid, hydrogen and carbon dioxide. The final digestion is performed by means of methane producing bacteria. At least two types of bacteria have so to be used for the best possible result. Acid producing bacteria act best in the low pH range of about 5.0-6.0 and methane producing bacteria best in the pH range of 7.0-7.5. Except that acid producing bacteria act best in the lower pH range than methane producing bacteria they are neither so sensitive to fluctuations under the digestion conditions as methane producing bacteria.
Wastewaters containing biologically degradeable organic material have this far been treated among other things in socalled completely mixed reactors, in which acid and methane reactions take place in the same mixed space. Because methane producing bacteria are much more sensitive to the fluctuations under the digestion conditions than acid producing bacteria, the conditions in these kinds of completely mixed reactors must be defined according to the requirements for methane producing bacteria, whereat acid reaction rates have not of course been the best possible in these kinds of completely mixed reactors.
It is known before how to digest anaerobically wastewater sludge, which contains biologically degradeable organic material, in two stages in a digestion reactor in which both stages are fitted inside each other. E.g. from the Finnish Pat. No. 57579 is known a device for digesting wastewater sludge in two successively coupled stages fitted inside each other, whereat sludge water and digested sludge are removed from a second stage, from which a part of sludge is fed back into a first stage. In this kind of a reactor the digestion cannot, however, be performed separately by means of two various types of bacteria, because these automatically would mix with each other when the portion of sludge from the second stage is fed as inoculation sludge into the first stage. This device is thus actually a completely mixed reactor, even though it is divided into two stages inside each other.
It is obvious that the disadvantages caused by the completely mixed reactors can be eliminated by using two reactors coupled to a series whereat the first one of the reactors would act as an acid stage and the second one as a methane stage. This kind of a device were, however, relatively expensive and complicated in comparison with the completely mixed reactor, and in addition it would require more space.
The object of the present invention is thus to provide a device for treatment of wastewater by means of anaerobic fermentation, in which device the acid and methane reactions can be performed under the conditions best adapted to these reactions without essentially mixing acid and methane producing bacteria with each other, and which device is, however, in broad outlines as cheap and simple as the completely mixed reactor.